Motorized pipettes of the prior art commonly are driven by stepper motors to insure that the motor operates within fixed, well-defined ranges without overtravel. However, such stepper motors are less readily obtainable in small sizes at low cost than the simpler D.C. motor. Furthermore, the stepper motor usually requires more complex drive circuitry than the D.C. motor. On the other hand, the D.C. motor suffers the disadvantage of overtravel because of inertia when the power to the motor is shut off. If the motor is connected to a screw to convert rotary motion to linear motion, overtravel is likely to lock up the screw against a fixed stop. In the past, therefore, pipettes run by D.C. motors have had to include controls, usually electrical, that prevent the overtravel from occurring in the first place. Examples include, e.g., feedback control circuitry as in German OLS No. 3,136,777, published Aug. 19, 1982, wherein a potentiometer controls the degree of motor activation, based upon piston position. Such controls in turn introduce complications and sources of unreliability so that a dilemma is created that causes the potential advantages of using a D.C. motor to be largely lost.
What has been needed prior to the present invention is a motorized pipette that uses an overtraveling motor and yet avoids the complicated controls heretofore used to prevent overtravel of the motor.